Vacuum feeds linked together. Good idea?

Hi all,

The question's about a 1987 Kwak gpz400r, which is a liquid cooled
four cylinder four stroke. It's got a vacuum switched fuel tap, and so
takes a vacuum feed from the inlet rubbers. This is done in a bit of a
weird way. The inside pair of cylinders have the vacuum feeds linked
together into a T-piece, the other pipe in this junction then goes to
the fuel tap. The outside two cylinders have a single length of pipe
connecting their vacuum feeds together, doing nothing else. I can't
decide (and don't know) what the point of all this is, or if it is
effective in doing anything. Surely it's not to compensate for if the
carbs get out of sync or something? The jets on the outside carbs are
different to those of the middle two, so I understand why all four
aren't linked together.

What I propose to do is sync the carbs properly, blanking off the four
vacuum feeds and to change the fuel tap for a standard (non-vacuum
shutoff) type. If this is a really bad idea, please can someone warn
me? Anyway, I know you guys have seen it all on this thread, so it'd
be nice to get a better idea of what this is all about.

Cheers for any help you can offer,

Adam

 

The point is that all the carbs are identical, right? It makes no
sense to manufacture some carbs with vacuum feeds and some without, as
it just puts up the production costs. So you blank offthe vacuum feeds
on the carbs that aren't connected to the fuel tap.

Also, are you *sure* the outside carbs are jetted differently to the
inside? I'd be surprised if that were the case.

Nothing wrong with using an ordinary gravity feed fuel tap, by the
way. Understand, though, that the vacuum take-offs for the fuel tap
are not the same as the vacuum take-offs for carb synching. Those are
separate.

 

It's not uncommon to find the inner two cylinders are jetted slightly
larger to compensate for running slightly hotter. RGV250s run a larger
jet in the left (rearward facing) cylinder for this reason too.

 

OK, fairy nuff. I've come across bikes with different plug heat ranges
on the same engine, so I suppose I shouldn't be surprised at different
jetting.

 

Often is - the middle two cylinders run hotter and often have a
different jetting or needle position.